Merck said on Monday its blockbuster cancer drug Keytruda failed to meet the main goal of a late-stage study testing the drug as a standalone treatment in patients with an aggressive type of breast cancer.
Shares of the drugmaker fell 0.8% to $78.27, in extended trading.
Keytruda, among a class of medicines called PD-1 inhibitors, did not meet the main goal of helping patients live longer, when compared to chemotherapy.
The treatment will continue to be studied in earlier stages of the disease and in combination with chemotherapy in patients with triple negative breast cancer, the company said.
Triple negative breast cancer occurs in about 10% to 20% of breast cancer patients and is more difficult to treat, the company said.
Keytruda, which brought in sales of $2.27 billion for the company in the first quarter, has U.S. approval for other forms of cancer including skin and lung cancer.